1. Field of the Invention
This invention relates to a piezoelectric resonator used for an intermediate frequency filter or an oscillator in a radio apparatus and a piezoelectric filter using this piezoelectric resonator.
2. Description of the Prior Art
Piezoelectric resonators used for conversion between a mechanical energy and an electric energy are widely known. A rectangular bar type piezoelectric resonator using the length expander mode and a ladder filter including such piezoelectric resonators are disclosed in Japanese utility model application provisional publication No. 55-138824. Such a prior art ladder filter comprises a plurality of stages of resonator circuits, each stage including a resonator connected in series (hereinafter referred to as a series resonator) and a resonator connected in parallel (hereinafter referred to as a parallel resonator). A passband of this ladder filter is obtained by agreement between a resonance frequency of the series resonator and an antiresonance frequency of the parallel resonator and maximum elimination bands are provided on the both sides of the passband. An amount of attenuation of this ladder filter is increased by increase in the number of stages and by increase in a ratio of capacitances between the series resonator and the parallel resonator. That is, to increase the amount of attenuation, the capacitance in the series resonator is made small by forming a partial electrode covering a surface of rectangular bar of the series resonator partially. Moreover, to increase the amount of the attenuation, namely, to reduce the capacitance in the series resonator, piezoelectric materials having different coupling factors or different dielectric constants should be used. This technique is disclosed in Japanese patent application provisional publication No. 50-67054.
An example of a prior art resonator will be described with reference to FIG. 7A. FIG. 7A is a perspective view of the prior art resonator used for the series resonator of a ladder filter. This prior art piezoelectric resonator comprises a rectangular piezoelectric bar 1 having a length L, a width W smaller than the length L, and a thickness T; a partial electrode 8 formed on the surface 2 defined by L.times.W; and a whole electrode 9 covering the opposite surface 3 defined by L.times.W. Reference P denotes a direction of polarization. FIG. 7B is a perspective view of the prior art piezoelectric resonator R11 used for the parallel resonator in the prior art ladder filter. The difference between the resonator shown in FIG. 7B from the resonators shown in FIG. 7A is the whole electrode 4 is used instead of the partial electrode 8.
However, in such a prior art rectangular bar type of piezoelectric resonators as shown in FIG. 7A and 7B, there is a problem that spurious responses caused by vibrations in the width and thickness directions is considerable large. Moreover, there is a problem that an interval in frequency between the resonance frequency and antiresonance frequency of the piezoelectric resonator is small, which is caused by providing a partial electrode 8 for reducing the capacitance in the series resonator. Further, there is a problem that a characteristic of the resonator is changed by dispersion of a location of the partial electrode 8 developed in the mounting processing of the partial electrode 8 or by dispersion of an area of the partial electrode 8.
Moreover, when dimensions of the prior art resonator as shown in FIG. 7A or 7B are determined to have a ratio between the length L and width W such that (W/L)&gt;(1/3), a coupling factor K31 of the length expander mode is considerably small.
In the filter including the prior art resonator disclosed in Japanese patent application provisional publication No. 50-67054, there is a problem that different materials having different coupling factors should be used between the series and parallel resonators. Moreover, in a filter using the series resonator having the partial electrode, there is a problem that a passband of the filter is made more narrow or there is a dispersion in a characteristic of the filter.
Another prior art ladder filter including series resonators and parallel resonators is disclosed in Japanese utility model application provisional publication No. 51-10878. FIGS. 15A and 15B are perspective views of prior art parallel and series resonators of this prior art ladder filter. The series resonator is formed to have a width W.sub.2 and a length L.sub.2. The parallel resonator is formed to have a width W.sub.1 and a length L.sub.1. The widths W.sub.1 and W.sub.2 are determined as to agree with each other to eliminate a spurious output caused by vibration in the width direction.
In these prior art resonators, there is a problem that peak spurious output caused by vibration in the width direction are steep and large.